The present disclosure relates to pressure sensor devices, and particularly to devices that identify changes in fluid pressure.
Pressure sensors typically measure absolute or relative pressure of fluids such as gasses or liquids. Measurement of fluids enables accurate control and monitoring of various devices and systems. There are various sensor devices using any of multiple available mechanisms of action for measuring pressure. For example, given sensor devices can use piezoelectric, piezoresistive, optical, electromagnetic, and other technologies for measuring pressure. Some pressure sensors are being manufactured at very small sizes. For example, microelectromechanical systems (MEMS) are now being used as pressure sensors. The main function of a MEMS sensor is to transfer a pressure signal into an electrical output signal based on an absolute or differential pressure input combined with offering an electrical base signal. Such relatively small pressure sensors are useful in systems where size and weight are valid considerations.
Conventional mid-pressure pressure sensors use metal alloy pedestals bonded to a MEMS die using a strong and stiff epoxy or eutectic bonding. Such metal alloy use is expensive and can compromise accuracy because of a significant difference in coefficient of thermal expansion values between the metal alloy material and the MEMS die. Conventional MEMS dies can also be manufactured with an integrated glass pedestal attached to the MEMS die during the wafer fabrication process. Such integrated glass pedestals (also referred to as fused glass pedestals) are relatively expensive, and the MEMS die is conventionally bonded to the glass pedestal using anodic bonding techniques, and the pedestal is bonded to a pressure port mounting frame by a bond or eutectic metal bond.
In conventional devices chemically resistant epoxy or eutectic bonding must provide mechanical bonding strength and a sufficient seal for transmission applications. It is difficult to provide cost effective, chemically robust and mechanically strong solutions, when components and adhesives are in contact with harsh media such as fuels, transmission fluids and oil.
Using MEMS technology in pressure sensing technologies can be beneficial because of the small footprint of MEMS devices relative to conventional pressure sensing technologies. As such, MEMS devices are suitable for applications having size constraints such as automotive transmissions including dual clutch transmission (DCT) systems with increasing requirements for smaller sensors. A challenge with MEMS pressure sensing devices, however, is creating a cost-efficient as well as a robust assembly or package. Conventional MEMS pressure sensors can be relatively expensive and lose accuracy or even fail under certain operating conditions including high temperature, thermal expansion, and chemical deterioration. For example, conventional MEMS pressure sensors are costly due to the number and type of parts, and the use of precious metals such as gold for improving bonding capabilities. A MEMS sensing element should be sealed chemically from a sensed media (such as fluid or oil pressure). Also, with conventional components made of varied materials, there is a mismatch in coefficients of thermal expansion (CTE). The result of such a mismatch is that thermal conditions can break down a chemical seal due to thermal stress build-up and/or apply excessive expansive stress to a MEMS sensing element.
One challenge with using MEMS pressure sensing elements is determining how to attach a MEMS die to a substrate to result in a sufficiently strong connection (physical attachment), but while enabling accurate pressure readings. A strong mechanical connection and good accuracy can conflict when thermal expansion coefficients of the components are not matched. With a strong connection there is always a corresponding level of stress that can be undesirable. For example, temperature changes at either relatively high or low temperatures can induce stress on a MEMS die attachment bond. A rigid connection translates these stresses to the die, affecting accuracy of pressure measurement.